This invention relates to ultrasound diagnostic imaging systems and, in particular, to a method and apparatus for rapidly obtaining extended field of view ultrasound images.
Diagnostic ultrasound systems are commonly used to generate two-dimensional (xe2x80x9c2-Dxe2x80x9d) and three-dimensional (xe2x80x9c3-Dxe2x80x9d) images of tissues, vessels and organs within a patient""s body. To do so, a sonographer positions an ultrasound scanhead having an array of transducer elements adjacent to a target area. The transducer elements emit ultrasound energy that propagates into the patient where it is absorbed, dispersed, refracted, and reflected by internal structures. Reflected ultrasound energy is received back at the scanhead where it is converted back into electronic signals. An image is then created from the electronic signals.
The received electronic signals undergo beamforming to coordinate the samples in time and space to a target area. Exemplary beamforming methods for controlling the imaging process include focus, steering, apodization and aperture. Focus is a time delay profile of active transducer elements. Steering is the control of focus depth points along azimuth and elevation axes of the transducer elements. Apodization is a voltage weighting profile of active transducer elements. Aperture is the control of the number of transducer elements that are active along an axis of the scanhead. The beamformed signals are processed to display an image showing echo and Doppler flow information, which may be in the form of a cross-sectional image.
A conventional cross-sectional image is a brightness image (i.e., referred to as a xe2x80x9cB-modexe2x80x9d or xe2x80x9cB-scanxe2x80x9d image) in which component pixels are brightened in proportion to the intensity of a corresponding echo signal. Existing B-scan ultrasound imaging systems use scanheads having one-dimensional linear arrays to generate B-scan images of the body. The images produced by B-scan ultrasound imaging systems are composed of discrete image frames, the characteristics of which depend on the number of transducer elements that are active, the relative spacing of the elements and the steering and focus of the transducer elements. Each B-scan image frame represents a two-dimensional (xe2x80x9c2Dxe2x80x9d ) image plane that is taken through a cross-section of the body that extends inwardly from the linear transducer array.
A drawback of such B-scan imaging is that most of the imaged tissues or vessels appear only as cross sections since most tissues or vessels of interest do not extend along the image plane. It is therefore often difficult using B-scan imaging to visualize tissues or vessels extending through the body at approximately a constant distance from a skin surface with which the scanhead is in contact.
One approach to making B-scan imaging more useful is to combine a large number of 2-D image frames to create an xe2x80x9cextended field of viewxe2x80x9d (xe2x80x9cEFOVxe2x80x9d) or xe2x80x9cpanoramicxe2x80x9d image. In these systems, the scanhead is moved along a skinline to produce successive 2-D B-scan image frames that represent respective spatially offset 2-D image planes, as explained above. Each image plane is defined by a centerline of the scanhead array, i.e., the path along which the ultrasound is directed, and a direction that extends along the axis of the transducer array. The scanhead is scanned in a direction extending along the axis of the array to create a series of 2-D B-scan image frames. The image frames lie in a common plane and have regions that spatially overlap each other. The image frames are then combined by registering the overlapping areas of adjacent image frames. The resulting image is a 2-D EFOV B-scan image lying in a plane extending in the scanning direction. Alternatively, the scanhead may be scanned in a direction that is perpendicular to the axis of the array to create a series of B-scan image frames that lie in different planes that are parallel to each other. The image frames are obtained sufficiently close to each other that beam patterns of the frames spatially overlap each other in elevation. The image frames are then combined by registering the adjacent image frames. The resulting image is a 3-D EFOV B-scan image containing all of the B-scan image frames.
In order to make proper registration of the image frames possible, accurate information about the distance between adjacent frames must be known. Early EFOV imaging systems, known as xe2x80x9cB-arm scanning systems,xe2x80x9d included a single beam ultrasound scanhead mounted at the end of an articulated arm. The joints of the articulated arm contained sensors that produced an electrical signal indicative of the spatial position of the scanhead. As the scanhead was scanned over the body of the patient, an image frame was produced from the ultrasound returns obtained from the scanhead and the relative spatial locations of the scanhead while the returns were being obtained. The image frames from multiple adjacent scans of the scanhead were computed and stored, and then assembled in consecutive, side-by-side locations to create an EFOV image. These early EFOV systems were capable of generating an ultrasound image that could laterally extend for the maximum number of successive image frames that the system could store and display and extend vertically over the range of positions that arm could extend.
EFOV imaging systems relying on hardware position sensors have several shortcomings. First, position sensors based on electromagnetic energy emissions may interfere with the transmitted and received ultrasound energy. Other hardware position sensors tend to be less accurate requiring longer and more frequent calibration processes. Also, it is a challenge to integrate the sensor""s detection scheme into the ultrasound image capturing process. The position sensor captures data samples. Such samples need to be synchronized to the ultrasound sampling process and the ultrasound data processing data. Finally, EFOV imaging systems having scanheads mounted at the end of an arm are cumbersome to operate because the arm tends to restrict freedom of movement.
In recent years, systems have been developed for electronically registering B-scan images to produce an EFOV image. As previously explained, the scanhead in these systems is scanned along a skinline to produce successive, spatially offset 2-D image frames, . Each image frame is spatially registered with a previously acquired overlapping image frame, and the image frames are then combined to produce an EFOV image that is laterally extensive in the direction of motion of the scanhead.
One conventional technique for producing a 2-D EFOV B-scan image is shown in FIG. 1. An ultrasound scanhead 10 having a linear array of transducer elements 12 is placed in contact with a skinline 14 of a patient. The ultrasound scanhead 10 is coupled to an imaging system (not shown in FIG. 1) by a cable 16. In the example shown in FIG. 1, the ultrasound scanhead 10 is being used to scan tissues 20 beneath the skinline 14 containing a blood vessel 24 that divides into two branches 26, 28 at one end. However, it will be understood that the ultrasound scanhead 10 can likewise be used to scan other blood vessels as well as tissues, vessels or organs.
To scan a length of the blood vessels 24, 26, 28, the sonographer slides the ultrasound scanhead 10 in the direction 30. With reference, also, to FIG. 2, as the ultrasound scanhead 10 is moved in the direction 30, successive 2-D B-scan image frames 34, 36, 38 lying in substantially the same plane are acquired. Each of the image frames 34, 36, 38 is composed of data from ultrasound echoes returned from all locations in a thin volume represented by the image frame. Each image frame 34, 36, 38 is slightly displaced from the previous image frame in the direction 30. The magnitude of the image frame displacement is a function of the speed the scanhead 10 is moved and the rate at which image frames 34, 36, 38 are acquired. As explained in greater detail below, the displacement between successive image frames 34, 36, 38 is computed and the image frames are registered and combined on the basis of the displacements to produce a 2-D EFOV B-scan image of the tissues 20 and blood vessels 24, 26, 28. It is therefore important for adjacent image frames 34, 36, 38 to overlap each other at least slightly so that they can be properly registered with each other by suitable means, such as cross-correlation techniques.
The image frames 34, 36, 38 are individually shown in respective FIGS. 3A-C. As shown in FIG. 3B, the image frame 36 overlaps the image frame 34 starting at point A, and it overlaps the image frame 38 starting point C. In practice, the image frames 34, 36, 38 would generally overlap each other to a greater degree than shown in FIG. 2, but doing so in FIG. 2 would make it difficult to visualize the individual image frames 34, 36,38.
Ideally, it is desirable for the ultrasound scanhead 10 to be translated at a constant speed while image frames 34, 36, 38 are being acquired so that individual image frames 34, 36, 38 are not stretched or compressed laterally relative to earlier acquired image frames 34, 36, 38. It is also desirable for the scanhead 10 to be moved in a single plane so there is high correlation from each image frame 34, 36, 38 to the next. However, manual scanning over an irregular body surface often causes departures from either or both of these desirable conditions. Either or both of these effects of less than desirable manual scanning can be compensated for by conventional means. It will also be understood that image frames 34, 36, 38 can be obtained using an ultrasound scanhead that is structurally different from the ultrasound scanhead 10 shown in FIG. 1.
The adjacent image frames are typically registered with each other by using a cross-correlation algorithm to identify corresponding structures in each image frame. The corresponding structures may be patterns in tissues or vessels, or may be speckle present in the 2-D images. Speckle results when an ultrasound beam transmitted into the body is scattered by microstructures that are too small to be resolved by the ultrasound beam, i.e., approximately smaller than the wavelength of the ultrasound. Although the microstructures are too small to be resolved by the ultrasound beam, the microstructures nevertheless disperse, reflect, or otherwise interfere with the signal that is returned to the scanhead. When an image is created based on the returned ultrasound signal, this interference, which is noise known as xe2x80x9cspecklexe2x80x9d causes the image to appear granular. As shown in FIGS. 3A-C, each of the image frames 34, 36, 38 contain speckle 40, in addition to the vessels, tissues or blood flow being imaged. The speckle 40 appearing in each image frame 34, 36, 38 is substantially identical for corresponding locations in the underlying tissues 20 since the speckle 40 is caused by stationary microstructures, as previously explained. Therefore, the speckle 40 is present in each of the image frames 34, 36, 38 at locations that are spatially offset by the movement of the scanhead 10 from one image frame 34, 36, 38 to the next. The speckle 40 can then be used to properly register the image frames 34, 36, 38 with each other, as shown in FIG. 4. Adjacent image frames 34, 36, 38 can be properly registered with each other by suitable techniques, such as using a cross-correlation algorithm to identify corresponding structures in each image frame. The adjacent image frames are then registered with each other by electronically placing the corresponding structures in the same position. Although the speckle 40 is shown in FIGS. 3A-3C as being in only one location in each of the image frames 34, 36, 38, it will be understood that much of the image frame will normally contain some speckle.
One problem with EFOV imaging systems using electronic registration results from the time required to process the image frames 34, 36, 38 to determine proper registration. Cross-correlation algorithms typically used to properly register the image frames 34, 36, 38 are computationally intensive and thus require a substantial period of time even when using high-speed processors. The time required for the cross-correlation algorithm to properly register adjacent image frames 34, 36, 38 limits the frame rate, i.e., the speed at which image frames can be acquired. Limiting the frame rate, in turn, limits the speed at which the scanhead 10 may be scanned in order to acquire an image. As a result, it can take a substantial period of time to acquire an image using EFOV imaging systems. Furthermore, it can be difficult for even a trained operator to move the scanhead 10 at the proper speed. Moving the scanhead 10 too quickly can result in insufficient overlap between adjacent image frames 34, 36, 38 to properly register the image frames. Moving the scanhead 10 too slowly only serves to further increase the considerable time needed to acquire an EFOV image.
The time required to acquire an EFOV image could be reduced by reducing the number of 2-D image frames 34, 36, 38 that are combined to create the EFOV image. However, reducing the number of 2-D image frames 34, 36, 38 used to form the EFOV image can seriously degrade the quality of the resulting EFOV image.
Although the problem with conventional EFOV imaging systems has been explained with respect to 2-D EFOV B-scan images formed by combining 2-D Bscan image frames, it also exists when forming a three dimensional (3-D) EFOV image. For example, the rate at which a 3-D EFOV image can be formed by combining 3-D image volumes is also limited by the time needed to properly register the 3-D image volumes. As another example, it also requires a great deal of time to properly register 2-D Doppler image frames or 3-D Doppler image volumes used to form a 3-D EFOV Doppler image.
There is therefore a need for a system and method for allowing image frames to be rapidly acquired and registered, thereby allowing high-quality EFOV images to be quickly obtained, particularly when producing 3-D EFOV images.
A method and system for displaying an extended field of view image includes an ultrasound scanhead that is scanned across a target area. While the target area is being scanned, data are acquired corresponding to substantially the entire portion of each image frame or volume in a first set of spatially overlapping ultrasound image frames or volumes. During the scan, data are also acquired corresponding to a relatively small part of each image frame or volume in a second set of spatially overlapping ultrasound image frames or volumes that are interspersed with the image frames or volumes in the first set. Speckle that is present in at least the image frames or volumes in the second set is then used to determine the displacement of the scanhead from respective positions where the data for each of the image frames or volumes are acquired. Based upon these displacement determinations, data corresponding to the image frames or volumes in the second set are processed to create image data corresponding to the image frames or volumes in the first set combined and registered with each other. This image data corresponds to an extended field of view image that can be then displayed.